1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with a unique attachment joint for burnable absorber rods which allows easy detachment and consolidation of the spent absorber rods.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles.
Also, it is conventional practice to design an excessive amount of neutron flux into the reactor core at start-up so that as the flux is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. In view of this practice, in some reactor applications burnable absorber or poison rods are inserted within the guide thimbles of some fuel assemblies to assist the control rods in the guide thimbles of other fuel assemblies in maintaining the neutron flux or reactivity of the reactor core relatively constant over its lifetime. The burnable poison rods, like the control rods, contain neutron absorber material. They differ from the control rods mainly in that they are maintained in stationary positions within the guide thimbles during their period of use in the core. The overall advantages to be gained in using burnable poison rods at stationary positions in a nuclear reactor core are described in U.S. Pat. Nos. 3,361,857 to Rose and 3,510,398 to Wood.
With the increased use of an 18-month fuel cycle in operation of nuclear reactors in contrast to the more usual 12-month cycle, the need to consolidate spent burnable absorber assemblies, in an efficient manner, has become an important factor in the overall reactor fuel servicing. Previously with the use of the 12-month fuel cycle, burnable absorber assemblies were only required in the first cycle and so the spent absorber assemblies could be stored with the spent fuel assemblies. Now, in the 18-month fuel cycle, burnable absorber assemblies are required in the second and third cycles of the fuel. Thus, there are more spent absorber assemblies generated than there are spent fuel assemblies so all of the spent absorber assemblies cannot be stored in spent fuel assemblies. This means that the absorber rods must be consolidated to save space in the spent fuel pool.
The present design of the burnable absorber cluster assembly, for example as illustrated and described in the patent application cross-referenced above, does not lend itself to consolidation since the upper ends of the rods are fastened to a base or support plate of the assembly with threaded connections which are locked in place. To remove the rods from the support plate it is necessary to shear the rods therefrom which is not an easy task to perform remotely while the irradiated assembly is submerged in water.
Consequently, a need exists for a different approach to attaching the absorber rods to the support plate which will still ensure that the rods are held securely in their desired positions within the guide thimbles during use in the fuel assembly in the reactor core, but after being used up will allow easy detachment of the spent rods from the support plate in order to facilitate their efficient consolidation in the spent fuel pool.